Flavonoid Biosynthesis in Scutellaria baicalensis Georgi: Metabolomics and Transcriptomics Analysis

Scutellaria baicalensis Georgi (SB), a plant of the Lamiaceae family, contains flavonoids with potent human health benefits. The full mechanistic details and regulatory networks related to the biosynthesis of these compounds in SB have been the focus of recent research but are still fragmented. Similarly, a complete account of the metabolites produced, specifically flavonoids, and their distribution in different parts of the plant is incomplete. To provide a more complete picture, herein we have explored the SB metabolites and differentially expressed genes in underground and aerial tissues. Of the 947 metabolites identified, 373 were differentially accumulated flavonoids (DAFs), and 147 of these were differentially accumulated in roots relative to other tissues. Interestingly, roots accumulated more baicalin and baicalein than aboveground tissues, but they were low in scutellarein and wogonoside, in contrast to previous reports. These differences may be attributed to either plant variety, age of the plants, or the extraction protocol. Transcriptomics analysis identified 56 key genes from the flavonoid synthesis pathway in all six SB plant tissues. A weighted gene correlation network analysis conducted using four DAFs (baicalin, baicalein, scutellarein and wogonoside) produced 13 modules. Baicalin and baicalein were positively correlated with one of these modules, whereas wogonoside and scutellarein were correlated with three other modules. Gene expression in these modules was consistent with the observed accumulation of these compounds in plant tissues. Fourteen structural genes were highly correlated with baicalin, baicalein and scutellarein, and 241 transcription factors (TFs) associated to these four compounds. The 13 highly correlated structural genes and 21 highly correlated TFs were used to construct correlation networks, where genes were identified to be highly correlated with flavonoid biosynthesis genes. Overexpression of some of these genes, namely, SbMYB8 (Sb02g25620), SbMYB14 (Sb09g00160) and SbbHLH94 (Sb07g11990), in SB callus increased flavonoid content and regulated the expression of genes involved in the flavonoid biosynthesis pathway, confirming their association to flavonoid production. Overall, the present work contributes to delineating the differences in flavonoid biosynthesis among different SB tissues.